In an image acquisition device for observing a sample such as a tissue cell, when the distance between the sample on the stage and an objective lens is kept constant, irregularities on a surface of the sample may have an out-of-focus region in an image. Therefore, image acquisition devices employing various focusing methods such as a dynamic focus scheme which captures an image of the sample while acquiring focus information and a prefocus scheme which acquires focus information before capturing the image of the sample have been developed.